Conventionally, a high temperature superconducting filter is used for communication of mobiles, communication of satellites, etc. In this case, the high temperature superconducting filter is used such that it is installed in the interior of a cooler to be cooled at a temperature of approximately 70K (Kelvin). And, the filter in the cooler is connected to a machine positioned on the outside of the cooler by the semi-rigid cable. Accordingly, it is necessary to suppress a heat inflow amount which is inflow from a room temperature to a cold stage (a low temperature portion by the cooler) through the semi-rigid cable, in order to lower a load of the cooler, or make it possible to use a cooler which is of a smaller type and a lower cooling capability, and lighter.
In case of making a long semi-rigid cable to be used to lower a heat inflow amount, or an outer diameter of it small, however, it is not preferable because transmission loss of an electromagnetic wave signal is increased. Further, in case of changing a material of a conductor, although a slight improvement is obtained, it is resulted that loss of an electromagnetic wave signal is increased, as a heat inflow amount is decreased, because a thermal conductivity of a metallic material is basically proportional to an electrical conductivity thereof in accordance with the law of Wiedemann-Franz.
Under such technical background, it is developed for a semi-rigid cable connecting between low and room temperature circumstances that an outer conductor (a conductor provided on an outer circumferential side of a coaxial cable) which is most related to heat inflow is fabricated by plating a thin film of a copper which is well in conductivity on an outside of a dielectric layer of fluoro-resin, as seen in products of Cryodevice Inc. According to this method, a thickness of copper which is an outer conductor is approximately 10 μm, so that it has a sufficient thickness not to invite the increase of loss, because a surface skin of, for instance, copper at 2 GHz (depth necessary for the transmission of signals) is approximately 1 μm. Further, a thickness of an outer conductor of an ordinary semi-rigid cable is more than 0.1 mm, so that a thickness of an outer conductor is made thin by approximately 10%, and a heat inflow amount coming trough the outer conductor is decreased by 10%.
Further, there is “a coaxial cable” disclosed in Japanese Patent Application laid-open No. 9-12904 as a prior application's invention example 1 similar in technical filed to the present invention. This has a double structure of an outer conductor comprising an outside outer conductor of bad thermal conductivity and an inside outer conductor of well electrical conductivity, so that electrical conductivity is ensured, and thermal transmission is suppressed from the outside of the cable to the inside thereof.
In the conventional semi-rigid cable, however, heat is easily transmitted from the exterior of a cold stage (a low temperature portion such as the interior of a cooler) to the interior thereof, because, for instance, copper which is a well conductor and well at thermal transmission is used for an outer conductor, and the outer conductor has a sufficient thickness to consider mechanical strength.
Further, there is a problem in reliability in a semi-rigid cable of Cryodevice Inc. in that a thin outer conductor is especially to be easily cracked or broken in bending process, so that a conductive plane is easily cut. Further, when a tough cable is used in consideration of mechanical strength and durable years, there occurs a problem in that costs increase in ensuring cooling force and an electric power bill for a cooler.
Further, as clearly described in section [0012] of the prior application's invention example 1, the outside outer conductor has no relation with signal transmission, and a purpose of the outside outer conductor is for the suppression of heat transmission into the inside outer conductor. That is, the purpose is for the suppression of the heat inflow toward the inside, so that it is not appropriate for a measure against a heat inflow flowing in the longitudinal direction of a cable or through a cross-section of a cable as intended by the present invention.
Explaining in more concretely, the outside outer conductor is desired to prevent heat from flowing to the inside outer conductor to be as thick as possible in accordance with the purpose of the prior application's invention example 1. For instance, when a stainless steel having a thickness of approximately 1 cm is used, it works largely as a non-thermal conductor to easily provide a temperature difference from several degrees to several tens degrees, although it deviates depending on balance of a heat inflow amount. In the invention, however, thermal shielding in the lengthwise of a cable, that is, a heat inflow through a cable cross-section is suppressed, the cable cross-section is preferable to be thin even at a portion of a non-thermal conductor. In a coaxial cable in the prior application's invention example 1, heat becomes difficult to be flowed from outside to inside, and mechanical strength is ensured, so that the outside outer conductor is preferable to be thick. That is, heat is made easier to be flowed in the longitudinal direction of a cable from the exterior of a cold stage to the interior thereof, and cost becomes high in a cooler. As described above, a semi-rigid cable according to the present invention is not along the object of the prior application's invention example 1, and the prior application's invention example 1 does not solve a problem of the present invention.